THE ROLE OF CAFFEINE-SENSITIVE CA2+ STORES IN AGONIST-INDUCED AND INOSITOL 1,4,5-TRISPHOSPHATE-INDUCED CA2+ RELEASE FROM BOVINE ADRENAL CHROMAFFIN CELLS

In single bovine adrenal chromaffin cells loaded with fura-2, histamine, angiotensin II (A(II)) and caffeine elicited large transient increases of intracellular free Ca2+ concentration ([Ca2+]i) in the absence of external Ca2+, with peak amplitudes averaging 726 +/- 138 (n = 14), 710 +/- 102 (n = 21) and 830 +/- 100 nM (n = 30) respectively. A substantial portion of the agonist-induced rise in [Ca2+]i dependend on Ca2+ release from caffeine-sensitive stores, as pretreatment with caffeine diminished subsequent agonist responses by 90-95 %. Conversely, pretreatment with histamine or A(II) decreased subsequent caffeine responses by 100 % and 90 % respectively. The effects of caffeine most likely resulted from activation of a Ca2+-induced Ca2+-release (CICR) process, whereas histamine and A(II) initially acted through generation of Ins(1,4,5)P3. The relationship of Ins(1,4,5)P3- and caffeine-sensitive Ca2+ pools was studied by using alpha-toxin-permeabilized chromaffin cells. Evidence was found for three non-mitochondrial, ATP-dependent, Ca2+ pools: one exclusively sensitive to Ins(1,4,5)P3 (pool 1), a second sensitive to both Ins(1,4,5)P3 and caffeine (pool 2), and a third exclusively sensitive to caffeine (pool 3). The existence of pools 1 and 3, and the ability of agonists such as histamine to discharge pool 3 completely, supports a two-pool model in which a caffeine-sensitive CICR mechanism plays a major role in the generation of agonist-induced Ca2+ spikes in bovine chromaffin cells.